Non-invasive imaging technologies allow images of the internal structures of a patient or object to be obtained without performing an invasive procedure on the patient or object. In particular, technologies such as computed tomography (CT) use various physical principals, such as the differential transmission of x-rays through the target volume, to acquire image data and to construct tomographic images (e.g., three-dimensional representations of the interior of the human body or of other imaged structures).
The radio density (also referred to as the CT attenuation or the CT number), of each voxel of the image data is represented by a numeric value along an arbitrary scale, the Hounsfield scale, which may be defined so that −1000 represents the radio density of air and +1000 represents the radio density of bone. Air causes very little x-ray attenuation and is typically depicted in black on x-ray films, in CT images, and so on, whereas bone greatly attenuates x-rays and is typically depicted in white on these films and images. Fat has a radio density of about −120 Hounsfield units (HU), and muscle has a radio density of about +40 HU. Water is defined as having a radio density of 0 HU.
Intermediate amounts of CT attenuation are usually depicted by shades of gray in CT images. Because the human eye is unable to distinguish among 2000 shades of gray (representing HU values between −1000 and +1000), a radiographer selects a range of CT attenuations that is of interest (e.g., a range of HU values, known as a “window”), and all the CT attenuations within this range are spread over an available gray scale, such as 256 shades of gray. This mapping of a range of CT attenuations to shades of gray is known as windowing. The center of the range is known as the window level. Typically, materials having radio densities higher than the top of the window are depicted in white, whereas materials having radio densities lower than the bottom of the window are depicted in black.
It is difficult to appropriately map images with a large dynamic range with a straight-forward application of the windowing process described above. Currently, operators of an imaging system can adjust the window of an image to view structures within that window only, but cannot simultaneously view multiple windows in a single image. For example, it may be difficult to image bones, organs, and soft tissue in a single image.